Gas sensor, especially a lambda probe

ABSTRACT

To improve the electrical properties of the gas sensor, the following measures may be used:  
     In the top view of the layer planes of the body of the sensor, printed circuit traces for electrodes are situated outside of cavities in the body, in particular outside of the reference air duct.  
     The electrodes are enlarged in the direction of the exhaust-side end of the sensor.  
     The printed circuit traces have an increased layer thickness or are formed as a double layer.

BACKGROUND INFORMATION

[0001] The present invention relates to gas sensors, in particular lambda probes, having a body formed as a sintered ceramic laminate and a reference air duct situated therein within a layer of the laminate, an electrical resistance heater being provided on its one side and an electrode configuration being provided on its other side, the electrode configuration having at least one reference electrode that is situated on the inside of a boundary wall of the reference air duct and is permeable for gases at least regionally and that has a Nernst electrode that is acted upon by gases to be sensed, the Nernst electrode also being at least regionally permeable for gases and being separate from the reference electrode by a solid electrolyte layer that is conductive and permeable for ions, oxygen ions in particular, and the electrodes being connected to printed circuit traces that essentially extend in the direction of the reference air duct.

[0002] Today, exhaust systems of modern internal combustion engines, particularly for motor vehicles, are regularly provided with catalytic converters for converting harmful exhaust gases into harmless reaction products. In order for the catalytic converters to function well, it is necessary to feed air and fuel to the engine in a predefined proportion. The engine controls provided for this purpose are connected on their input side to a so-called lambda probe whose signals represent the composition of the exhaust gas and, thus, enable the engine control to adjust the ratio of fuel and combustion air in a manner optimal for the catalytic converter.

[0003] Two designs are known in this connection.

[0004] In the one design, stoichiometric combustion is targeted, i.e., the oxygen quantity in the combustion air corresponds exactly to the oxygen requirement for complete combustion of the supplied fuel. Therefore, the engine is operated using neither an excess of oxygen (λ>1) nor using a deficiency of oxygen (λ<1). This operating method may, therefore, by characterized by λ=1.

[0005] When sensing exhaust gas, narrow-band lambda probes where the Nernst electrode is acted upon by the exhaust gas as directly as possible are sufficient for this stoichiometric combustion.

[0006] In this instance, the effect is used by the engine control that an electrical voltage able to be tapped off between the reference electrode and the Nernst electrode and generated by diffusion of oxygen ions significantly changes its value in the vicinity of λ=1, and a signal is consequently available that clearly displays a deviation from the desired operating mode using stoichiometric combustion in the direction of an operating mode having an oxygen deficiency as well as in the direction of an operating mode having an oxygen excess.

[0007] Such sensors are shown in DE 44 01 749 A1, for example.

[0008] In the other design, the objective is predominant operation of the internal combustion engine with an oxygen excess during combustion since the fuel consumption is able to be noticeably reduced as a result. However, during combustion using an oxygen excess, harmful nitrogen oxides are produced that may only be absorbed for a limited time by so-called adsorption catalysts in the exhaust branch of the motor vehicle. In each case prior to exhausting the absorption capacity of the adsorption catalysts, the engine operation must be switched over briefly to combustion with an oxygen deficiency in order for the incompletely combusted fuel components now entering the exhaust branch to be able to reduce the nitrogen oxides previously stored in the catalytic converter to nitrogen. In this instance, the engine control, i.e., the internal combustion engine, must be constantly switched at intervals between an operating mode that is predominant with respect to time and in which the values of λ are greater than 1 and a relatively brief operating mode in which the values of λ are less than 1.

[0009] Broadband lambda probes are necessary for such an intermittent operating mode having drastically changing values of λ.

[0010] In the case of such lambda probes, the Nernst electrode is situated at a separate chamber that communicates with the exhaust-gas stream via a diffusion path situated in the body of the probe. Situated within this chamber is also an internal pump electrode that may be electrically connected to the Nernst electrode and also cooperates through a solid electrolyte layer with an external pump electrode that is exposed to the exhaust-gas stream as directly as possible. If an external electrical voltage is applied between the two pump electrodes, which are both designed to be permeable for gases at least regionally, an oxygen ion current whose direction depends on the polarity of the applied voltage and whose intensity is determined by the electrical voltage difference as well as by the difference in the oxygen concentration at the pump electrodes is generated between the pump electrodes. This oxygen ion current accordingly controls the diffusion current of the exhaust gases in the diffusion chamber. The external electrical voltage between the pump electrodes and the electrical current occurring between the pump electrodes due to the oxygen ion current are adjusted by a controller so that an electrical voltage having a predefined setpoint value is always maintained between the reference electrode and the Nernst electrode. As such, the polarity and intensity of the electrical current occurring between the pump electrodes are a signal that correlates to the composition of the exhaust gases and, thus, to the λ values.

[0011] Such probes are represented in DE 37 44 206 A1, for example.

[0012] Aging processes such as pollution change the properties of the aforementioned gas sensors.

SUMMARY OF THE INVENTION

[0013] In accordance with the present invention, it is provided that in a top view of the layer planes of the laminate, the printed circuit traces corresponding to the electrodes are situated at least partially next to the reference air duct.

[0014] This is particularly applicable for the printed circuit traces of the pump electrodes.

[0015] The present invention is based on the general idea of using the pressing pressure exerted during and/or prior to sintering the probe body to the laminate for compressing the composite structure of the printed circuit traces and in this connection to effectively increase the pressure forces exerted on the printed circuit traces by situating the printed circuit traces in the laminate without being covered by hollow spaces as viewed from above. Consequently, a smaller electrical resistance of the printed circuit traces as well as a higher durability of the printed circuit traces with respect to aging effects is achieved with the result that the change in the electrical properties of the probe in the case of increasing age are significantly reduced.

[0016] In addition or alternatively, further measures may be provided. For example, the internal and/or external pump electrode may have a surface that is larger than the base plan of the gas chamber situated between the Nernst electrode and the internal pump electrode.

[0017] In this context, it has proven to be particularly advantageous when the pump electrodes on a region diametrically opposed to the corresponding printed circuit trace, i.e., in the direction of the top end of the probe body, extend beyond the base plan of the gas chamber.

[0018] Furthermore, it has proven to be advantageous for constant electrical properties when the printed circuit traces have a comparably large layer thickness. For this purpose, the printed circuit traces may be produced using printing technology with relatively wide-meshed screens (e.g. screens having a 250 mesh). Moreover, the printed circuit traces may also be printed as a double-layer.

[0019] Finally, the pressure material for the printed circuit traces may have a particularly high proportion of electrically conductive particles, in particular based on platinum.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Exemplary embodiments of the lambda probe according to the present invention are explained in greater detail below and are shown in the drawings.

[0021] The figures show:

[0022]FIG. 1 shows a cross section of a broadband lambda probe corresponding to line of intersection I-I in FIGS. 2 and 3 in the region of the top end of the probe body projecting into the exhaust-gas stream.

[0023]FIGS. 2 and 3 show top views corresponding to arrows II and III in FIG. 1 of different layers of the laminate having electrodes as well as corresponding printed circuit traces.

[0024]FIG. 4 shows a longitudinal section of part of the probe corresponding to line of intersection IV-IV in FIG. 1.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0025] According to FIG. 1, the represented lambda probe has a body 1, which is formed as a ceramic laminate. The layers of the laminate are placed or deposited on one another in the green condition. Subsequent sintering, which may be carried out after or during simultaneous pressing of the laminate, produces a hard ceramic body 1.

[0026] In the example in FIG. 1, a bottom layer 2 is provided in the form of a thicker film of zirconium oxide. Above this is an electrically insulating double layer 3, in which an electrical resistance heater 4 as well as corresponding printed circuit traces for electrical current supply are embedded. Above that lies layer 5, which is produced and patterned by screen printing and is made, for example, of a zirconium oxide paste. Recessed within this layer is a reference air duct 6, whose base plan is shown by way of example in FIG. 2 and is explained in greater detail below. As shown, this reference air duct 6 may have two end regions 6′, which communicate with one another, in the area of the sectional plane in FIG. 1.

[0027] In some instances, layer 5 may also be formed by a film in which duct 6 is punched out.

[0028] Above layer 5 is a solid electrolyte layer 7, e.g. in the form of a film made of zirconium oxide to which yttrium oxide is added. A gas-permeable, layered reference electrode 8 of a porous platinum material, which is connected via a layered printed circuit trace 8′ connected thereto (see FIG. 2) to a connection contact on body 1 explained below, is situated on the side of layer 7 facing reference air duct 6, i.e., between layers 5 and 7, at least in the area of end regions 6′ of reference air duct 6.

[0029] Above solid electrolyte layer 8 is a thin layer 9, which is patterned using printing technology and is in turn produced from a zirconium oxide paste, for example. This layer 9 has a large recess that is situated centrically to an exhaust-gas access hole 10, which extends through body 1 perpendicularly to its layers. Porous material 12 is deposited within the indicated recess while leaving open a ring space 11. As shown, access hole 10 may be designed as a blind hole or, deviating from the representation, as an opening passing completely through body 1.

[0030] In the region of annular space 11, solid electrolyte layer 7 supports a gas-permeable, layered Nernst electrode 13 of a porous platinum material.

[0031] Another solid electrolyte layer 14, e.g. in the form of a film made of zirconium oxide to which yttrium oxide is added, lies above layer 9 and porous material 12. This layer 14 supports on it side facing annular space 11 as well as on its side away from annular space 11 gas-permeable, internal and external pump electrodes 15 and 16 made of an at least regionally porous platinum material, these electrodes 15 and 16 being formed such that in a top view of the layers of body 1, they at least essentially cover annular space 11. A gas-permeable protective layer 17 lies above layer 14.

[0032] In some instances, a positive image of reference air duct 6 as well as of its end pieces 6′ and orifices 6″ may also be imprinted on layer 7 using a material that is disintegrated or burned off when sintering body 1 or forms a porous, effectively gas-permeable structure.

[0033] In general, it is also possible to print layer 3 in a mirror image to layer 7 using the material of layer 5 and, in some instances, also using the material provided for the positive image of reference air duct 6 and it parts 6′ and 6″. In this manner, layer 5 is able to be produced with a greater thickness.

[0034] The previously described lambda probe functions as follows:

[0035] The end of body 1 having exhaust-gas access hole 10 is situated in the exhaust-gas stream or in a region communicating with the exhaust-gas stream of an internal combustion engine, while the other end of body 1 is acted upon by reference air typically from the atmosphere.

[0036] Reference air reaches end pieces 6′ of the reference air duct via reference air duct 6 and its orifices 6″. Via exhaust-gas access hole 10, exhaust gas reaches porous material 12, through which the exhaust gas diffuses into annular space 11.

[0037] When the exhaust gas-side end of body 1 is sufficiently heated by electrical resistance heater 4, an electrical voltage, whose magnitude depends on the partial oxygen pressures within end pieces 6′ of the reference air duct as well as within annular space 11, is able to be tapped off between reference electrode 8 and Nernst electrode 13 and consequently between plated through-holes 19 and 20. In this context, the effect is taken advantage of that solid electrolyte layer 7 conducts oxygen ions and the platinum material of aforementioned electrodes 8 and 13 promotes or enables the formation of these oxygen ions with the result that an electrical potential difference that is dependent on the partial oxygen pressure at electrodes 8 and 13 and results in a corresponding ion migration occurs in solid electrolyte layer 7. This potential difference is also known as the Nernst voltage.

[0038] The partial oxygen pressure in annular space 11 is able to be controlled in that an external electrical voltage having controllable polarity is applied between pump electrodes 15 and 16. The corresponding voltage source is connected to plated through-holes or contacts (not shown) that are electrically connected to pump electrodes 15 and 16.

[0039] In this instance, the effect is in turn used that the platinum material of electrodes 15 and 16 results in the formation of oxygen ions and an oxygen ion current flowing through solid electrolyte layer 14 and having an intensity and direction dependent on the electrical voltage and its polarity is then produced by the external electrical voltage between electrodes 15 and 16. Thus, an electrical signal is able to be tapped off between pump electrodes 15 and 16, e.g. is able to be determined by ascertaining the voltage and current intensity of the electrical resistance of the electric circuit leading across the pump electrodes.

[0040] The electrical voltage and consequently also the electrical current between pump electrodes 15 and 16 are controlled by a controller such that electrical voltage able to be tapped off between reference electrode 8 and Nernst electrode 13 and consequently the partial oxygen pressure in annular space 11 always correspond to a defined setpoint value. Therefore, the electrical current able to be tapped off between pump electrodes 15 and 16 is a measure of the oxygen content of the exhaust gas relative to the reference air.

[0041] When external pump electrode 16 is at an electrically positive potential with respect to internal pump electrode 15, operating conditions where λ>1 exist. In the case of a reverse polarity, operating conditions of λ<1 exist, the magnitude of the electrical resistance between electrodes 15 and 16 correlating to the magnitude of λ.

[0042] The values of λ may be acquired in a large value range.

[0043] In the case of the narrow-band lambda probe indicated at the outset, external protective layer 17 is above Nernst electrode 13, i.e., layers 9 and 14 as well as pump electrodes 15 and 16 are not necessary in comparison with the representations in FIGS. 1 and 3. Given a known and constant partial oxygen pressure, the electrical voltage able to be tapped off between electrodes 8 and 13 is a measure of the partial oxygen pressure of the exhaust gases.

[0044]FIG. 2 shows a top view corresponding to arrow II in FIG. 1 of solid electrolyte layer 7. Reference electrode 8 as well as a printed circuit trace 8′ connected thereto are imprinted on the side of this layer 7 visible in FIG. 2. This printed circuit trace 8′ leads to a plated through-hole (not shown), which is able to extend, for example, through layer 7 as well as the layers above layer 7 in FIG. 1 and to electrically connect printed circuit trace 8′ to a contact tag (not shown) situated externally on body 1 on its reference air-side end.

[0045] Furthermore, a dashed line in FIG. 2 represents the position of reference air duct 6 including its ends 6′ situated in the region of reference electrode 8.

[0046] In FIG. 2, printed circuit trace 8′ is outside of reference air duct 6. Consequently, printed circuit trace 8′ is subjected to an increased pressing pressure when the laminate of body 1 is pressed prior to and/or during sintering in order to ensure good cohesion of the layers of the laminate.

[0047] While reduced pressing pressures occur during this pressing above and/or below reference air duct 6 because almost no forces are able to be applied over the hollow space of duct 6, a high pressure is always ensured in regions next to reference air duct 6 because in this instance there are no considerable cavities in the laminate.

[0048] The aforementioned increase in pressing pressure at printed circuit trace 8′ is especially effective when reference air duct 6 is produced within a film-like layer 5 by punching.

[0049] Deviating from the representation in FIG. 2, it may also be provided that instead of fork-shaped end regions 6′ of the reference air duct, only one single end piece enlarged in some instances with respect to the rest of reference air duct 6 be situated below gas access opening 10, which in this case must be designed as a blind hole in order to be able to ensure a separation from reference air duct 6. Reference electrode 8 has a form similar to the aforementioned end piece of the reference air duct such that reference electrode 8 covers the aforementioned end piece of reference air duct 6 with more or less excess.

[0050]FIG. 3 shows a top view of solid electrolyte layer 14 corresponding to arrow III in FIG. 1. Dotted lines represent the position of annular space 11 as well as of porous material 12 via which annular space 11 communicates with gas access hole 10.

[0051] External pump electrode 16 is imprinted on the side of solid electrolyte layer 14 visible in FIG. 3. It has an ring-shaped design similar to the ring shape of annular space 11. However, external pump electrode 16 may be significantly enlarged particularly in the direction of the exhaust-side end of layer 14 and also may project in the direction of the longitudinal sides of layer 14 beyond the borders of annular space 11.

[0052] As a result of this design, a good pump effect is able to be achieved already at a low voltage between pump electrodes 15 and 16, it being ensured at the same time that a clear proportionality results between the lambda values and the pump current.

[0053] Internal pump electrode 15 may have a shape similar to external pump electrode 16.

[0054] Moreover, it may be provided for porous material 12 to be situated in regions adjacent to large-area zones of internal and external pump electrodes 15, 16, respectively, having a narrower width in the radial direction to gas access opening 10. In this manner, the gas access to annular chamber 11 in zones in which pump electrodes 15 and 16 have an increased pump effect is made easier.

[0055] It may be provided for all electrodes to design the corresponding printed circuit traces with particularly good electrical conductivity.

[0056] For example, this may be achieved in that the material used for printing the printed circuit traces has an increased platinum content or an increased content of other effectively electrically conductive particles. While the electrodes must be permeable for gas and are therefore produced using printing technology with a particle mixture that, during sintering, forms an electrically conductive layer that is permeable for gases and ions, gas permeability does not need to be ensured for the printed circuit traces. Accordingly, the metal content of the material of the printed circuit traces may be increased. For example, the electrode material may contain a high proportion of zirconium oxide in comparison with the platinum content, while the material of the corresponding printed circuit traces has a low zirconium oxide content in comparison with the platinum proportion.

[0057] A further possibility for increasing the electrical conductivity of the printed circuit traces is to provide an increased layer thickness of the printed circuit traces. This may be achieved, for example, in that the printed circuit traces are produced by screen printing using comparably wide-meshed nets.

[0058] Furthermore, there is the possibility of producing printed circuit traces in each case as a double layer, i.e., two conductive layers may be situated or printed one above the other.

[0059]FIG. 4 shows a longitudinal section corresponding to line of intersection IV-IV in FIG. 2 through layers 7, 9, and 14.

[0060] Nernst electrode 13, which is situated on layer 9 and above chamber 11 and whose form may be similar to pump electrodes 15 and 16 as viewed from above, has an assigned printed circuit trace 13′, which borders layer 9 and is coincident with printed circuit trace 15′ of internal pump electrode 15, so that printed circuit traces 15′ and 13′ form a double layer.

[0061] To enable such a configuration, layer 14 may first be coated on its bottom side in FIG. 4 only right of a border G with the material of layer 9. Electrode 15 and corresponding printed circuit trace 15′ are subsequently printed, a connection between printed circuit trace 15′ on layer 9 and electrode 15 on layer 14 being automatically created in the region of border G.

[0062] Porous material 12 as well as the still missing part of layer 9 are then imprinted or superposed on layer 14.

[0063] Pump electrode 13 as well as corresponding printed circuit trace 13′ may be imprinted on layer 7, which is subsequently placed on layer 9, printed circuit traces 15′ and 13′ being sintered together when the laminate is later sintered.

[0064] Deviating from the representations in FIGS. 3 and 4, in particular when reference air duct 6 is produced within layer 5 by punching, printed circuit traces (e.g. 13′, 15′, 16′) of all electrodes are situated off-center on the appropriate layers of the laminate such that, in the top view of the layer planes, no covering by reference air duct 6 is able to occur, and pressing the laminate makes it possible to compact the material of the printed circuit traces particularly effectively while improving the electrical conductivity as was explained above by way of example using FIG. 2 for printed circuit trace 8′ of reference electrode 8. 

What is claimed is:
 1. A gas sensor, in particular a lambda probe, having a body (1) designed as a sintered ceramic laminate and a reference air duct (6) situated therein within a layer (5) of the laminate, an electrical resistance heater (4) being provided on one side of the reference air duct and an electrode configuration (8, 13; 15, 16) being provided on its other side, the electrode configuration having at least one internal reference electrode (8), which is situated on a border wall of the reference air duct (6) and is at least regionally permeable for gases, and a Nernst electrode (13), which is able to be acted upon by the gas to be sensed, is also at least regionally permeable for gases, and is separated from the reference electrode (8) by a solid electrolyte layer (7), which is conductive and permeable for ions, in particular oxygen ions, the electrodes (8, 13; 15, 16) being connected to printed circuit traces (8′, 13′, 15′, 16′), which essentially extend in parallel to the reference air duct (6), wherein in a top view of the layer planes of the laminate, the printed circuit traces (8′, 13′, 15′, 16′) are situated at least partially next to the reference air duct (6).
 2. The gas sensor, in particular a lambda probe, having a body (1) designed as a sintered ceramic laminate and a reference air duct (6) situated therein within a layer (5) of the laminate, an electrical resistance heater (4) being provided on one side of the reference air duct and an electrode configuration (8, 13; 15, 16) being provided on its other side, the electrode configuration having at least one internal reference electrode (8), which is situated on a border wall of the reference air duct (6) and is at least regionally permeable for gases, and a Nernst electrode (13), which is able to be acted upon by the gas to be sensed, is also at least regionally permeable for gases, and is separated from the reference electrode (8) by a solid electrolyte layer (7), which is conductive and permeable for ions, in particular oxygen ions, in particular as recited in claim 1, wherein the Nernst electrode (13) is situated at a separate chamber (11), which communicates with the exhaust-gas stream via a diffusion path (12) situated in the body (1), an internal pump electrode (15) is situated within this chamber (11), the internal pump electrode cooperating via a solid electrolyte layer (14) with an external pump electrode (16), which is exposed to the exhaust-gas stream as directly as possible, and in a top view of the layer planes of the laminate, the Nernst electrode (13) and/or the internal and/or the external pump electrode (15, 16) extends beyond the borders of the chamber (11).
 3. The gas sensor as recited in claim 2, wherein outside of the chamber (11), in a top view, at least one of the pump electrodes (15, 16) and/or the Nernst electrode (13) has a large-area region that extends in the direction of the exhaust-side, top end of the body (1).
 4. A gas sensor, in particular a lambda probe, having a body (1) designed as a sintered ceramic laminate and reference air duct (6) situated therein within a layer (5) of the laminate, an electrical resistance heater (4) being provided on one side of the reference air duct and an electrode configuration (8, 13; 15, 16) being provided on its other side, the electrode configuration having at least one internal reference electrode (8), which is situated on a border wall of the reference air duct (6) and is at least regionally permeable for gases, and a Nernst electrode (13), which is able to be acted upon by the gas to be sensed, is also at least regionally permeable for gases and is separated from the reference electrode (8) by a solid electrolyte layer (7), which is conductive and permeable for ions, in particular oxygen ions, in particular as recited in one of claims 1 through 3, wherein the printed circuit traces (8′, 13′, 15′, 16′) connected to electrodes (8, 13; 15, 16) have increased electrical conductivity with respect to the electrodes.
 5. The gas sensor as recited in claim 4, wherein the printed circuit traces have an increased metal or platinum content with respect to the electrodes.
 6. The gas sensor as recited in claim 4 or 5, wherein the printed circuit traces are configured with an increased thickness with respect to the electrodes.
 7. The gas sensor as recited in one of claims 4 through 6, wherein the printed circuit traces are formed as a double layer. 